Diamond has a number of properties which make it attractive for use in various applications. Diamond has the highest thermal conductivity of any known material, and is an electrical insulator, so it is an ideal heat sink material. Other useful properties are extreme hardness, thermal stability, and excellent transmissivity of certain radiation. However, natural diamond is prohibitively expensive for applications which require any substantial size.
In recent years, a number of techniques have been developed for synthesizing diamond and for depositing synthetic diamond to obtain a diamond film or coating. These techniques include so-called high-pressure high-temperature ("HPHT") methods and chemical vapor deposition ("CVD") methods. The CVD methods include plasma deposition techniques wherein, for example, plasmas of a hydrocarbon and hydrogen are obtained using electrical arcing. The resultant plasma can be focused and accelerated toward a substrate using focusing and accelerating magnets. Reference can be made, for example, to U.S. Pat. Nos. 4,471,003, 4,487,162, and 5,204,144 for description of examples of a type of plasma jet deposition that can be utilized to deposit synthetic diamond on a substrate.
Synthetic diamond film can be deposited, for example, as a permanent coating on a substrate, such as on the wear surface of a tool or as an environmentally protective coating. Such films are generally considered to be relatively thin films. Alternatively, a synthetic diamond film that is generally considered a thick film, can be deposited on a substrate and then removed, preferably intact as a single "free standing" piece, for use in applications such as heat sinks, optical windows, and in tools. However, the obtainment of such thick films, especially of relatively large area, has proven troublesome. In addition to the difficulty of depositing quality synthetic diamond of substantial thickness, there is the problem of removing the diamond intact from the substrate. The substrate material will generally have a different coefficient of expansion than the diamond, as well as a different molecular and chemical structure. The adherence and growth of the diamond film, as well as its release, will depend, inter alia, on the materials used, surface preparation, and deposition parameters. Reference can be made, for example, to copending U.S. patent application Ser. No. 07/973,994, now U.S. Pat. No. 5,314,652 assigned to the same assignee as the present application, which discloses techniques for fabricating free-standing synthetic diamond films utilizing specified substrate roughnesses to help prevent premature lift-off of diamond film and to facilitate appropriate intact release thereof. As disclosed in the referenced copending Application, the substrate can include an interlayer (such as titanium nitride) which further facilitates the technique.
A further problem in CVD synthetic diamond deposition that needs to be addressed is the distortion (called bowing or curling) of the diamond film, particularly after its release from the substrate on which it has been deposited. (As used herein, "substrate" is intended to broadly mean any surface on which the diamond is deposited). It has been recognized that bowing can somehow result from intrinsic stress in the deposited diamond. Prior art (e.g. U.S. Pat. No. 5,270,077) indicates that diamond coatings grow in tension due to growth defects and the "intrinsic strain" therein is proportional to the coating thickness and also to the rate of deposition. The '077 Patent states that this intrinsic strain manifests itself by a bowing and/or cracking in the diamond film that has been released from a rigid substrate. After release, the diamond film relieves the tensile stress which was within the diamond coating by bowing into a curved configuration or by cracking. The '077 Patent indicates that the bowing that results upon release of the tensile stress in the diamond coating can be compensated for by growing the diamond coating on a convex growth surface such that, when the diamond coating is released from the substrate, the diamond coating will relieve the inherent tensile stress therein by bowing into a flat configuration without the formation of cracks or fragmentation. The '007 Patent further indicates that the diamond coating can be grown in a curved configuration on the substrate which is opposite to the direction of the stress-relieving deformation that results when the coating is released from the substrate. By matching the curvature of the substrate to the tensile stress created in a CVD diamond film as it grows, the '007 Patent indicates that stress-relieving deformation flattens the film when released from the substrate.
Applicant has found that the direction of bowing described in the '007 Patent generally does not occur in the CVD processes employed by Applicant, and that bowing generally occurs in the opposite direction, that is, with the deposition side (not the substrate side) bowing to a convex shape. Also, the behavior of diamond films indicates that a different understanding of the intrinsic stress phenomenon is required to account for and successfully address problems of bowing in relatively large area free-standing diamond films.
It is among the objects of the present invention to improve techniques for producing relatively large area free-standing diamond films, and to reduce stresses which produce distortions and other problems in the fabrication of such films.